Vacuum interrupters provide fast, low energy arc interruption with long contact life, low maintenance, low mechanical stress, and maximum operating safety. Modern vacuum interrupters, such as produced by the assignee of the present invention, employ vacuum switches having a metal and ceramic housing for maximum strength. A high alumina ceramic housing has more than five times the strength of glass. The metal end-closures and arcing chambers are of high purity alloy to minimize contamination. The vacuum switches are housed together in a tank-like enclosure along with high voltage components.
As a voltage potential is applied between two components, a level is reached at which the insulating medium (e.g., air, oil, SF.sub.6, etc.) cannot withstand the voltage stress and it flashes or arcs over. The withstand voltage level can be increased by spreading the components apart so that the voltage stress is reduced, or by using insulating medium with a higher voltage withstand level. The addition of insulation has the effect of increasing the voltage withstand level of the insulating medium. It does this because the barrier material has a higher withstand level than the surrounding insulating medium (e.g., air, oil, SF.sub.6, etc.). It breaks up the paths where the flash-over would occur by increasing the live part string clearance.
Dielectric insulating barriers appear in two common forms:
1. Sheets of material that are located between high potential part and ground, or between the adjacent phase parts. These are referred to as "phase barriers"; and PA1 2. Sheets that are attached to the interrupter tank or housing wall. These are referred to as tank or housing liners. PA1 1. It prevents dielectric voltage flash-over by increasing the insulating medium voltage withstand level. PA1 2. It facilitates interchangeability of new and existing vacuum switches in the same circuit interrupter. PA1 3. It provides a smaller and lower weight circuit interrupter because of the smaller dielectric clearances required. PA1 4. It provides for a more reliable circuit interrupter having more consistent voltage withstand levels. PA1 5. It is smaller in size and less costly than phase barriers or tank liners. PA1 6. It provides for more insulating coverage to include the leads, bushings and other live parts, instead of merely phase-to-phase or phase-to-ground protection. PA1 7. It results in a generally less costly circuit interrupter. PA1 8. It provides for efficient heat transfer to cool the vacuum interrupter assembly, inasmuch as it is not applied on the body of the associated high voltage conductor or the body of the high voltage vacuum switch.
Electrical breakdown is significantly promoted by temperature which is often high enough to change the physical structure of the dielectric. Such a high temperature may be caused by heat from an external source; however, localized heat generated by the passage of current through the dielectric itself can be a cause of breakdown.
The weak point in insulation systems often occurs at the interface between materials of different permittivities. This explains why an electric arc always runs over the surface of an insulator suspended in air, rather than penetrating the insulator or traveling a much shorter distance through the air. This weak point is alleviated if the difference between the two permittivities is small. Thus, a solid insulator immersed in oil experiences a lower surface stress than one immersed in air; however, oil has its disadvantages.
The technology of testing and maintaining solid dielectrics is further complicated by the long-term effects of partial discharge or corona. As voltages are pushed higher, extruded plastic dielectrics have been developed to take the place of oil impregnated paper. Nevertheless, partial discharge and subsequent treeing have added a new dimension to dielectric problems. On problem is that it is very difficult to manufacture a solid dielectric that is totally uniform. Corona is most likely to occur at a sharp point or irregularity on a high voltage conductor, because the voltage stress is greater at points where the conductor surface is sharply curved. Thus, the design of high voltage vacuum interrupters is by no means simple. When components are housed close together and when potentials are increased to match new customer demands, arcing and flash-over can occur. Flash-over is further complicated because small changes in component geometry and material act together to defy simple solution.